1. Field of the Invention
The present invention relates to utilizing a beam of light to detect the presence of objects, and more particularly to optical sensing equipment in which light from an emitter is returned by a retroreflector to a detector to create a light beam which is broken by objects to be detected.
2. Description of the Related Art
It often is desirable to detect the presence of objects moving along a conveyor line. This enables manufacturing and other types of equipment perform operations on the objects. Various object detection systems have been developed for this purpose.
One class of object detection systems are optical, using light to sense an object's presence. Within this optical class are reflex systems in which an emitter-detector assembly is located on one side of the path of the objects and a retroreflector, often called a target, is placed on the opposite side. A beam of light is sent from an emitter across the path to the retroreflector from which the light returns back across the path to a detector. An object moving along the path interrupts the beam of light, thereby providing an indication of the presence of the object.
Retroreflectors are sometimes called corner cube prisms or trihedral prisms because the reflecting surfaces are three mutually perpendicular faces like the corner of a cube. Light entering a retroreflector is reflected back 180° parallel to the incident light path, regardless of the reflector's orientation to the light beam. This kind of prism is designed to reflect any light ray or beam entering the prism back onto itself, even at very large angles of incidence. In comparison, a mirror only does that at the normal angle of incidence. This characteristic makes retroreflectors valuable for object detection, as the prism does not have to be precisely aligned with respect to the emitter-detector assembly, where precision alignment is difficult or time-consuming with other optical systems.
A reflex sensor should respond to light reflected from a retroreflector, but not respond to diffuse reflective objects, such as those being carried by the conveyor. The present technology accomplishes this in part by restricting the angle at which light is emitted by the light source and the field of view of the detector optics. These restrictions often require use of a relatively small retroreflective target and accurate sensor aiming, thus negating the principal advantage of retroreflectors, as precise aiming is not always easily accomplished in practical applications.
In another instance of present technology, the emitter-detector assembly has a coaxial design, such as shown in U.S. Pat. No. 3,996,476. Here, the emitter and detector devices are located one behind that other so that the emitted light path merges with the returned light path. This is accomplished with a lens of other optical element which directs the reflected light around the emitter onto the detector device behind. A disadvantage of this design is the requirement for careful mechanical alignment of the additional optical element. Furthermore, the optical and other components must be well designed, manufactured, and kept very clean to reduce internal light scattering. The lens and other optical elements of this system also attenuate the reflected light before reaching the detector thus degrading efficiency of the apparatus. The coaxial geometry is relatively sensitive to diffuse objects that are very close to the sensor. The resulting sensitivity to near objects and dirt on the lens is a significant practical disadvantage of prior coaxial emitter-detector assembly designs.
In another assembly the emitter and detector devices are arranged side by side with a beam splitter and a mirror to direct the emitted and received light along a common path between the emitter-detector assembly and the retroreflector. This system has some of the same disadvantages as the coaxial design.
Therefore, a need exists for an improved emitter-detector assembly for a retroreflective object detection system.